In the display method for liquid crystal display devices used in big screen televisions, passive matrix systems and active matrix systems are used, and the active matrix system is mainly used in applications requiring high image quality.
In the active matrix system, thin film transistors are connected to the individual pixels. Heretofore, the semiconductor layer of these thin film transistors are often formed of a-Si, but in the a-Si manufacturing process, because the vacuum process such as plasma CVD is used for forming a thin film, the process for film formation is complex and process management is difficult.
In order to solve these problems, much attention has been placed on the organic thin film transistor in which the semiconductor layer of the thin film transistor is formed of an organic semiconductor material. It has been proposed that the semiconductor layer of the organic thin film transistor is formed using a coating method such as the inkjet method or the spin coat method (For example refer to SID '02 Digest p 57). However, because the soluble organic semiconductor material that can be used in the coating methods have poor properties, there was a problem that sufficient thin film transistor properties could not be obtained.
In order to solve these types of problems, much attention has been given to the method which uses a semiconductor precursor. The semiconductor precursor is a substance which undergoes chemical changes when physical energy is applied and thereby becomes a semiconductor. There are some substances which do not dissolve in solvents in the semiconductor state, but dissolve in solvents in the precursor state. Known examples include precursors such as pentacene, oligothiophene, porphyrin which have excellent semiconductor properties. After these precursors are coated using a coating method, when the precursor is converted to the semiconductor, a semiconductor layer with excellent semiconductor properties can be formed by a simple process.
For example, a method has been proposed in which a photothermal conversion layer (combined with a gate insulation layer) is formed on a precursor that is formed on a glass substrate, and heat is generated in the photothermal conversion layer using heat from laser radiation, and the semiconductor precursor is thereby converted to a semiconductor (see Japanese Laid-Open Patent Publication No. 2004-146575).
However the organic thin film transistor element created by the method described in Japanese Laid-Open Patent Publication No. 2004-146575 was problematic in terms of reliability because the photothermal conversion layer remained on the substrate.
In addition, the properties of the thin film transistor depend to a large extent on the compatibility of the gate insulating layer and the semiconductor layer, but in the method described in the Japanese Laid-Open Patent Publication No. 2004-146575, it is necessary to select a material that has photothermal conversion functions as the gate insulating layer, and there are few options. For this reason, as the gate insulating layer and the semiconductor layer do not have excellent compatibility, sufficient properties as an organic thin film transistor element cannot be obtained.
Furthermore, after an electrode and the like are formed, the precursor is coated on the substrate and thus coating with a uniform film thickness was difficult. For this reason, in some case there was variation in the organic thin film transistor element properties.